1. Field of the Invention
This invention relates to magnetic thin film heads (TFH) for recording and reading magnetic transitions on a moving magnetic medium. In particular, the invention relates to a low-noise toroidal TFH device having low coil resistance and inductance, especially suitable for very high magnetic recording areal densities and channel frequencies. It is applicable to both conventional and planar inductive and to magnetoresistive (MR) heads using toroidal inductive write elements.
2. Background of the Prior Art
Magnetic TFH transducers are known in the prior art. See, e.g. U.S. Pat. Nos. 4,016,601; 4,190,872; 4,652,954; 4,791,719 for inductive devices and U.S. Pat. Nos. 4,190,871 and 4,315,291 for magnetoresistive (MR) devices.
In the operation of a typical inductive TFH device, a moving magnetic storage medium is placed near the exposed pole-tips of the TFH transducer. During the read operation, the changing magnetic flux from magnetized regions in the moving storage medium induces a changing magnetic flux in the pole-tips and the gap between them. The magnetic flux is carried through the pole-tips and yoke-shaped core and around spiraling conductor coil winding turns located between the yoke arms. The changing magnetic flux induces an electrical voltage across the conductor coil. The electrical voltage is representative of the magnetic pattern stored on the moving magnetic storage medium. During the write operation, an electrical current is caused to flow through the conductor coil. The current in the coil induces a magnetic field across the gap between the pole-tips. A fringe field extends into the nearby moving magnetic storage medium, inducing (or writing) a magnetic domain (in the storage medium) in the same direction. Impressing current pulses of alternating polarity across the coil causes the writing of magnetic domains of alternating polarity in the storage medium.
Magnetoresistive (MR) TFH elements can only operate in the read mode. The electrical resistance of an MR element varies with the direction of its magnetization orientation. Magnetic flux from the moving magnetic storage medium induces changes in this orientation. As a result, the resistance of the MR element to a sensing electric current changes accordingly. The varying voltage signal is representative of the magneatic pattern stored on the magnetic medium. An inductive element, optimized for writing, is used to record transitions in the magnetic medium.
In the manufacture of conventional TFH transducers for magnetic recording, a large number of devices are fabricated simultaneously by depositing and patterning various layers on a ceramic wafer. When completed, the wafer is cut (or diced) and machined into individual sliders each having at least one transducer. The main elements of a TFH inductive transducer, roughly in the order in which they are deposited, are the (alumina) undercoat, the bottom magnetic pole, the flux gap material to provide spacing between the bottom and top magnetic pole-tips, one or more levels of electrical conductive spiraling coil windings interposed within insulation layers and located between the yoke arm parts of the bottom and top magnetic poles, the top magnetic pole, elevated studs (or posts) for connecting the coil to bonding pads (above the overcoat), a thick (alumina) overcoat, and the bonding pads. In the case of an MR TFH device, the MR read element, along with its shields, electrical leads, and biasing films (such as soft adjacent layer and/or exchange bias layer) are usually fabricated prior to the fabrication of the inductive write element.
The prior-art design of an inductive TFH transducer includes top and bottom magnetic poles (layers), each comprising a pole-tip and a yoke arm usually made of the alloy NiFe (permalloy). The magnetic poles are connected through a back-via in the back side of one of the yoke arms. They are separated by a planar spiraling coil(s) and insulation layers in the yoke arm region, and by a thin gap layer between the pole-tips in the front of the device. A typical prior-art TFH device is shown in FIGS. 1 and 2 of U.S. Pat. No. 4,190,872 (Feb. 26, 1980) to Jones et al, and in the front cover of Data Storage journal, the September 1994 issue. The latter is a top-view color microphotograph of an actual prior-art TFH device. These figures illustrate some of the serious drawbacks of the prior-art TFH device. Since the back-via accommodates only a small fraction of the back-width of the yoke arms, it restricts the magnetic flux there, causing a full or a partial saturation (during write operations), and thereby impairing the device efficiency and overwrite capability. The magnetic layer inside the via consists of multiple domains in various orientations which are subject to extreme levels of stress and stress gradients. These increase the device susceptibility to magnetic noise, due to magnetic domain wall movements, through magnetostrictive interaction.
In addition, domain structure and orientation in the remaining back portions (to the sides of the via), as well as in the back-via itself, are ill-defined, raising the likelihood of Barkhausen, "popcorn", and/or "wiggle" noise occurrence. For example, the magnetic flux in the "wings" portions of the pole (at the back to the left and right of the via) during write operations, is normal to the hard-axis orientation of the pole layer. This results in domain wall movements during and after write (as well as read) operations. Such domain wall movements result in magnetic noise. Often, the (top) magnetic layer is used for making electrical connections to other features, such as for electrical leads connecting the coil to the studs. An example of such electrical lead is clearly seen in the color photograph of Data storage (the gray permalloy strip from the coil via to the left side of the coil). This is similar to coil lead 21 in FIG. 1 of U.S. Pat. No. 4,190,872, which is often constructed of plated permalloy (deposited during the plating of the top pole). Such portions of the magnetic circuit constitute open branches and loose ends having undesirable magnetic domains, orientations, and characteristics. These domains may backlash and relax at different times than the main core, thereby adding to the total device noise.
Planar heads, or planar silicon heads (PSH), is another inductive TFH design in which the various layers, as well as the air bearing rails, are formed in a major plane of the substrate. Upon completion of the wafer fabrication, individual sliders are diced from the wafer without any further throat lapping, slider machining, or rail definition. The magnetic core of the planar head has a general shape of a rectangular frame. The magnetic core frame includes an elongated bottom segment formed at the plane of the substrate, two pillars (or studs) normal to the bottom segment and connected to it on either side, and two top magnetic pole-piece segments overlaying and parallel the bottom segment. The two top pole-piece segments are separated by a gap. Each of the top pole-piece segments is connected to one magnetic pillar on its side opposite the gap. The top magnetic pole-piece segments include additional, narrower, pole-tips. These pole-tips are separated by a magnetic transducing gap. The transducing gap is thus located at the top of the magnetic core frame. The planar head further includes one or more layers of spiral coils, which are wound around each of the magnetic pillars.
The long planar spiral coil turns of the conventional TFH device (cf. FIG. 1 of U.S. Pat. No. 4,190,872 and the color photo of Data Storage and also FIG. 4(a) for a planar head and FIG. 5(a) for a conventional TFH device with MR and a flat spiral coil) are inefficient in coupling the magnetic flux in the core since they only wrap around a short fraction or segment of the core length (around the back-via). Also, most of each turn is located far from the magnetic core. According to a publication by N. Yeh in IEEE Trans. On Magnetics, Vol. MAG-18, No. 1, pp. 233-237, January 1982, the average turn efficiency is only about 65%. Furthermore, the long turns have large resistance and parasitic inductance which limit the attainable device frequency and aggravate thermal noise. The large coil resistance generates excessive heat during write operations. The excessive heat increases the device's Barkhausen, popcorn, and/or wiggle noise through magnetostrictive interaction of the magnetic core with thermal stresses. The latter are exerted by adjacent materials, having different thermal expansion coefficients, such as alumina and/or hard-baked insulation.
Another source for noise in the prior-art inductive TFH device is due to the seed-layer used for plating the magnetic poles. The magnetic properties of the seed-layer are often quite different from those of the plated magnetic layers. In particular, the magnetic orientation, coercivity, and hard axis anisotropy field of the NiFe seed-layer may be quite different from that of the plated NiFe layers. As a result, the different signal produced from the seed-layer (which is part of the magnetic core) is superimposed as a source of noise. Also, interfacial mechanical stress exerted between the seed-layer and the plated NiFe magnetic pole may contribute to the device's noise. Since the seed-layer usually consists of NiFe of somewhat different composition and microstructure than the plated alloy, the stress level and direction of the seed-layer can be different from that of the plated NiFe layers. The different stress may adversely affect the device noise through magnetostrictive interaction with the plated NiFe layers.
Prior art magnetoresistive heads use an inductive write element that is "merged" with the MR read element by sharing of a magnetic layer which serves both as a top shield for the MR read element, and as the bottom pole for the inductive write element. MR "combination" or "composite" heads comprise independent or separate MR shields and inductive write magnetic poles. The flat spiral coil of the inductive write element has a relatively small number of coil turns in order to reduce resistance and inductance. The latter is required for high data transfer rates. The output signal of an inductive head, when used in the read mode, is proportional to the number of coil turns. Therefore, the inductive write element of the MR head is inadequate for reading due to its relatively small number of turns. An MR head, combining an inductive write element with large number of turns but still having low inductance and resistance, will offer adequate read signal output, by the inductive write element, for servo pattern and/or data stored at large radii tracks on a spinning disk.